The present invention generally relates to thin-film photovoltaic devices (PVs). More specifically, it is directed to photovoltaic devices, e.g., solar cells, with structural designs to enhance photoconversion efficiency by optimizing optical geometry for use with optical concentrators. Further, the present invention is directed to a low cost fabrication process for making such photovoltaic devices.
Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Photovoltaic devices convert electromagnetic radiation into electricity. Solar cells. also known as photovoltaic devices, are specifically used to generate electrical power. PV devices are used to drive power consuming loads to provide, for example, lighting, heating or to operate electronic equipment such as computers or remote monitoring or communications equipment. These power generation applications also often involve the charging of batteries or other energy storage devices so that equipment operation may continue when direct illumination from the sun or other ambient light sources is not available.
The falloff in intensity of an incident flux of electromagnetic radiation through a homogenous absorbing medium is generally given by:
I=I0exe2x88x92xcex1xxe2x80x83xe2x80x83(1)
where I0 is the intensity at an initial position (at xxe2x88x920), xcex1 is the absorption constant and x is the depth from x=0. Thus, the intensity decreases exponentially as the flux progresses through the medium. Accordingly, more light is absorbed with a greater thickness of absorbent media or if the absorption constant can be increased. Generally, the absorption constant for a given photoconductive medium is not adjustable. For certain photoconductive materials, e.g., 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI), or copper phthalocyanine (CuPc), very thick layers are undesirable due to high bulk resistivities. By suitably re-reflecting or recycling light several times through a given thin film of photoconductive material the optical path through a given photoconductive material can be substantially increased without incurring substantial additional bulk resistance. A solution is needed, therefore, which efficiently permits electromagnetic flux to be collected and delivered to the cavity containing the photoconductive material while also confining the delivered flux to the cavity so that it can absorbed.
Less expensive and more efficient devices for photogeneration of power have been sought to make solar power competitive with presently cheaper fossil fuels. Therefore organic photoconductors, such as CuPc and PTCBI, have been sought as materials for organic photovoltaic devices (OPVs) due to potential cost savings. The high bulk resistivities noted above make it desirable to utilize relatively thin films of these materials. However, the use of very thin organic photosensitive layers presents other obstacles to production of an efficient device. As explained above, very thin photosensitive layers absorb a small fraction of incident radiation thus keeping down external quantum efficiency. Another problem is that very thin films are more subject to defects such as shorts from incursion of the electrode material. Co-pending U.S. patent application Ser. No. 09/449,800 entitled xe2x80x9cOrganic Photosensitive Optoelectronic Device With an Exciton Blocking Layerxe2x80x9d to Forrest et al. incorporated herein by reference describes photosensitive heterostructures incorporating one or more exciton blocking layers which address some of the problems with very thin film OPVs. However, other solutions are needed to address the problem of low photoabsorption by very thin films, whether the films are organic or inorganic photoconductors.
The use of optical concentrators, as known as Winston collectors. is common in the fields of solar energy conversion. Such concentrators have been used primarily in thermal solar collection devices wherein a high thermal gradient is desired. To a lesser extent, they have been used with photovoltaic solar conversion devices. However, it is thought that such applications have been directed to devices wherein photoabsorption was expected to occur upon initial incidence of light upon the active photoconductive medium. If very thin photoconductor layers are used, it is likely that much of the concentrated radiation will not be absorbed. It may be reflected back into the device environment, absorbed by the substrate or merely pass through if the substrate is transparent. Thus, the use of concentrators alone does not address the problem of low photoabsorption by thin photoconductive layers.
Optical concentrators for radiation detection have also been used for the detection of Cerenkov or other radiation with photomultiplier (xe2x80x9cPMxe2x80x9d) tubes. PM tubes operate on an entirely different principle, i.e., the photoelectric effect, from solid state detectors such as the OPVs of the present invention. In a PM tube, low photoabsorption in the photoabsorbing medium, i.e., a metallic electrode, is not a concern, but PM tubes require high operating voltages unlike the OPVs disclosed herein.
The cross-sectional profile of an exemplary non-imaging concentrator is depicted in FIG. 1. This cross-section applies to both a conical concentrator, such as a truncated paraboloid, and a trough-shaped concentrator. With respect to the conical shape, the device collects radiation entering the circular entrance opening of diameter d1 within xc2x1xcex8max (the half angle of acceptance) and directs the radiation to the smaller exit opening of diameter d2 with negligible losses and can approach the so-called thermodynamic limit. This limit is the maximum permissible concentration for a given angular field of view. A trough-shaped concentrator having the cross-section of FIG. 1 aligned with its y axis in the east-west direction has an acceptance field of view well suited to solar motion and achieves moderate concentration with no diurnal tracking. Vertical reflecting walls at the trough ends can effectively recover shading and end losses. Conical concentrators provide higher concentration ratios than trough-shaped concentrators but require diurnal solar tracking due to the smaller acceptance angle. (After High Collection Nonimaging Optics by W. T. Welford and R. Winston, (hereinafter xe2x80x9cWelford and Winstonxe2x80x9d) pp 172-175, Academic Press, 1989, incorporated herein by reference).
The present invention discloses photovoltaic device structures which trap admitted light and recycle it through the contained photosensitive materials to maximize photoabsorption. The device structures are particularly suited for use in combination with optical concentrators.
It is an object of this invention to provide a high efficiency photoconversion structure for trapping and converting incident light to electrical energy.
It is a further object to provide a high efficiency photoconversion structure incorporating an optical concentrator to increase the collection of light.
It is a further object to provide a high efficiency photoconversion structure in which the incident light is admitted generally perpendicular to the planes of the photosensitive material layers.
It is a further object to provide a high efficiency photoconversion structure in which the incident light is admitted generally parallel to the planes of the photosensitive material layers.
It is a further object to provide a high efficiency photoconversion structure utilizing generally conical parabolic optical concentrators.
It is a further to provide a high efficiency photoconversion structure utilizing generally trough-shaped parabolic optical concentrators.
It is a further object to provide a high efficiency photoconversion structure having an array of optical concentrators and waveguide structures with interior and exterior surfaces of the concentrators serving to concentrate then recycle captured radiation.
It is a still further object to provide a low cost method for making such highly efficient photovoltaic devices.